Sorbents for the oxidation and removal of mercury

ABSTRACT

A promoted carbon and/or non-carbon base sorbent are described that are highly effective for the removal of mercury from flue gas streams. The promoted sorbent comprises a carbon and/or non-carbon base sorbent that has reacted with and contains forms of halogen and halides. Optional components may be added to increase and/or preserve reactivity and mercury capacity. These may be added directly with the base sorbent, or in-flight within a gas stream (air, flue gas, etc.), to enhance base sorbent performance and/or mercury capture. Mercury removal efficiencies obtained exceed conventional methods. The promoted sorbent can be regenerated and reused. Base sorbent treatment and preparation methods are also described. Methods for in-flight preparation, introduction, and control of the active base sorbent into the mercury contaminated gas stream are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/201,595 filed on Aug. 29, 2008, which is a division of U.S.patent application Ser. No. 11/209,163, filed on Aug. 22, 2005 (now U.S.Pat. No. 7,435,286), which claims priority from provisional application60/605,640, filed on Aug. 30, 2004. The disclosures of U.S. patentapplication Ser. Nos. 12/201,595; 11/209,163; and 60/604,640 are herebyincorporated herein by reference to the extent appropriate.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to methods and materials for the removalof pollutants from flue gas or product gas from a gasification system.In particular, mercury is removed from gas streams generated during theburning or gasification of fossil fuels by highly reactive regenerablesorbents.

2. Background of the Invention

The combustion and gasification of fossil fuel such as coal generatesflue gas that contains mercury and other trace elements that originatefrom the fuel. The release of the mercury (and other pollutants) to theenvironment must be controlled by use of sorbents, scrubbers, filters,precipitators, and other removal technologies. Mercury is initiallypresent in the elemental form during combustion and gasification. Indownstream process sections, such as in the ducts and stack of acombustion system, some of the elemental mercury is oxidized. The amountthat is oxidized depends on the amount of acid gases present in the fluegas and other factors. Amounts of mercury vary with the fuel, butconcentrations of mercury in the stream of flue gas from coal combustionare typically less than 5 parts per billion (ppb). Large coal combustionfacilities such as electric utilities may emit a pound of mercury, ormore, a day. Mercury removal applications include, without limitation,flue gas from coal (or other fossil fuel) combustion, wasteincineration, product gas from gasification, as well as off gases frommineral processing, metal refining, retorting, cement manufacturing,chloralkali plants, dental facilities, and crematories.

Mercury Sorbent Technologies

Several types of mercury control methods for flue gas have beeninvestigated, including injection of fine sorbent particles into a fluegas duct and passing the flue gas through a sorbent bed. Fine-particleinjection sorbents include activated carbon, metal oxide sorbent, sodiumsulfide particles, and basic silicate or oxide sorbents. When particleinjection is employed, the mercury captured on the sorbent particles isremoved from the gas stream in a particulate control device such as abaghouse or electrostatic precipitator (ESP) and collected along withash particulate. The sulfide and basic silicate and oxide particles areeffective only for the oxidized mercury, and the metal oxide sorbentsexhibit slower capture kinetics than the carbon particles. Additionally,injection of fine carbon particles into the flue gas stream has beenonly partially successful in removing mercury, especially elementalmercury, where effective removal of only about 60% is attained for someapplications with a FF (fabric filter) to collect carbon and ash. Evenlower removal rates have been observed when an ESP is used to collectthe carbon because the contact time of the carbon with the gas is veryshort.

A major problem with existing carbon injection systems is that thesorbent is relatively unreactive toward mercury. Consequently, thesesorbents must be used in large amounts, at high sorbent-to-mercuryratios, to effectively capture the mercury. These sorbents tend to berelatively expensive and cannot be easily separated from the ash forregeneration and reuse. The collection of carbon in the ash also createssolid waste disposal problems, and the spent sorbent may contaminate thecollected ash, preventing its use in various applications.

One solution has been to add an oxidative sorbent comprising analuminosilicate material impregnated with a very heavy dosage of one ormore oxidative metal halides plus activated carbon. For example, referto Varma et al. (20070140940). However, the amounts of metal saltsrequired for Hg oxidation are generally relatively large and expensive.Also, several of the salts that can be used in such a process are highlytoxic. Although the metal salts are present for oxidation in thisprocess, activated carbon is essential for getting adsorption of the Hg.As such, there is no synergistic role for the aluminosilicates as theyappear to be only a support for the oxidizing salts.

Another approach has been the injection of aluminosilicate particulatesuch as bentonite, which contains neither oxidizing salts nor halogencomplexes with a Lewis base site, and thus lacks the more powerfuloxidizing capability of the said complexes as described in thisapplication. For example, see U.S. Pat. No. 7,413,719. Additionally, theinjection of an aluminosilicate (kaolin or metakaolin) containingcalcium hypochlorite which thermally decomposes to form halogen is alsoknown. For example, see U.S. Patent Application No. 20030103882. Thusthese and similar impregnated aluminosilicate technologies require timein flight at appropriate high temperatures to heat the impregnatedsalt(s) to generate an oxidation site. This clearly represents a kineticbarrier to activation in contrast to the extremely fast complexingreaction of the Lewis acid on the surface of the appropriate Lewis basesorbent described in the present patent. The kinetic barrier is only forheating up the calcium hypochlorite to decompose it to Cl atoms ormolecules. Halogen (bimolecular or atomic) would complex with carbon ornoncarbon at any lower temperature to form reactive oxidation sites.Also, halide would require a very high temp or strong acid to formreactive halogen or halogen complex.

Yet another approach is the injection of bentonite plus a metal sulfideand a metal salt, none of which is oxidizing to elemental mercury andwould require a slow thermal activation step. For example, see U.S.Patent Application No. 20070119300.

The injection of halogen or halogen precursors in a hot zone, followedby contact with an alkaline material in a wet or dry scrubber is anotherapproach known in the art. With such an approach, elemental mercury isclaimed to be oxidized by the halogen to Hg(II) which is collected bythe alkaline material in the scrubber. For example, see U.S. Pat. No.6,808,692 (Oehr), U.S. Pat. No. 3,849,267 (Hilgen), U.S. Pat. No.5,435,980 (Felsvang), U.S. Pat. No. 6,375,909 (Dangtran), U.S. PatentApplication No. 20020114749 (Cole), U.S. Pat. No. 6,638,485 (Iida), U.S.Patent Application No. 20030185718 (Sellakumar), U.S. Patent ApplicationNo. 20030147793 (Breen), and U.S. Pat. No. 6,878,358 (Vosteen). However,even though it is known to inject halogen forms at some stage of thecombustion process, such a process does not utilize a complexing methodon a sorbent surface for conducting the oxidation and capture. Further,the alkaline material is rapidly surface-coated by the largeconcentrations of acid gases, lowering its capacity for adsorption ofHg(II). It is also recognized that the halogen forms initiallyintroduced or generated are far more reactive to the largeconcentrations of SO₂ and moisture in the flue gas, and so gas-phasereactions of the halogens with Hg are hindered. In contrast, the presentinvention takes advantage of the Lewis acid complexes that rapidly formon the sorbent surface to effect the Hg oxidation, rather than rely ongas phase reactions for oxidation. Thus HCl, HBr, SO₂Br, and othergas-phase products all festoon the surface and promote the activity ofthe sorbent by forming complexes with the sorbent to form a promotedsorbent.

Accordingly, there remains a need for more economical and effectivemercury removal technology. This invention provides for cost-effectiveremoval of pollutants, including mercury, using sorbent enhancementadditives and/or highly reactive sorbents, with contact times of seconds(or less), and that may be regenerated and reused.

SUMMARY

The various embodiments of the present invention overcome the variousaspects of the deficiencies of the prior art and provide new andeconomical methods for the removal of mercury from the gases produced inthe utilization of fossil fuels.

A halogen/halide-promoted sorbent is described that is highly effectivefor the removal of mercury from flue gas streams. The sorbent comprisesany activated carbon and/or non-carbon compound, such as porous orvesicular felsic or basaltic materials, clay-based compounds, alkalinecompounds, calcium hydroxide compounds, sodium acetate compounds, and/orbicarbonate compounds, or a combination thereof. Optional secondarycomponents and alkali may be added to further increase reactivity andmercury capacity. Mercury removal efficiencies obtained exceed or matchconventional methods with added benefits such as reduced costs.Optionally, the promoted sorbent can be regenerated and reused. Sorbenttreatment and/or preparation methods are also described. Methods forin-flight preparation, introduction, and control of the sorbent,promoter and promoted sorbent into the mercury contaminated gas streamare described.

When a promoted or a non-promoted base sorbent reacts with elemental oroxidized mercury, a mercury/sorbent chemical composition is formed and,in the case of elemental mercury reacting with the promoted basesorbent, the mercury is oxidized. As discussed in further detail inother portions of the specification, the base sorbent may be either acarbon or a non-carbon material or a combination thereof. Additionally,the mercury/sorbent chemical composition may be comprised of covalentbonds, ionic bonds and/or chemical complexes between the promoted ornon-promoted base sorbent and the oxidized mercury. The Lewis basicgroups on the non-promoted base sorbent, or the non-promoted portions ofa promoted base sorbent, are available for reaction with the Lewis acidgroups of already existing oxidized mercury in the mercury containinggas. Thus, mercury may be removed from the mercury containing gas streamthrough the formation of multiple and various mercury/sorbent chemicalcompositions even within the same process. For example, multiple siteson a sorbent particulate can form multiple and various mercury/sorbentchemical compositions in the case where only a portion of the sites onthe base sorbent particulate are promoted.

In some embodiments, a carbon and/or non-carbon promoted sorbent and/ora combination thereof is provided comprising a sorbent structure thathas reacted with a promoter selected from the group consisting ofhalides, halogens, and combinations thereof, such that the reactionproduct is effective for the removal of mercury from a gas stream. Thecarbon sorbent comprises reactive Lewis acid groups/sites; thenon-carbon sorbent comprises reactive Lewis basic groups/sites.

In an embodiment, a promoted carbon and/or non-carbon sorbent isprovided wherein the base sorbent is selected from the group consistingof carbon, activated carbon, porous or vesicular felsic and basalticmaterials, clay-based compounds, alkaline compounds, calcium hydroxidecompounds, sodium acetate compounds, and/or bicarbonate compounds, or acombination thereof, with an average particle size similar to that offly ash produced from a thermal process (combustion or gasification) orgreater than that of fly ash produced such that it is physicallyseparable therefrom, and combinations thereof, and the promoter isselected from the group consisting of molecular halogens, Group V (CASnomenclature is used throughout) halides, Group VI halides,hydrohalides, and combinations thereof. In an embodiment, the promotedsorbent (carbon, non-carbon, or their combination) may have a mass meanparticle diameter such that it can be substantially separated byphysical means from entrained ash in the gas stream from which mercuryis to be removed. In an embodiment, the base sorbent (carbon,non-carbon, or their combination) may have a mass mean particle diametergreater than about 40 micrometers.

In another embodiment, the promoted sorbent comprises from about 1 toabout 30 grams of promoter per 100 grams of base sorbent. Anotherembodiment further comprises an optional secondary component comprisinga halogen or a hydrohalide such that the reactivity and mercury capacityof the promoted sorbent are enhanced.

In another embodiment, the concentration of the optional secondarycomponent on the finished sorbent is within the range of from about 1 toabout 15 wt % of the concentration of the promoter on the finishedsorbent.

In another embodiment, an optional alkali component may preferably beadded to provide a synergistic effect through combination of this alkaliwith the base sorbent.

In another embodiment, the optional secondary component is selected fromthe group consisting of Group V halides, Group VI halides, HI, HBr, HCl,and combinations thereof. In another embodiment, the promoter issubstantially in vapor form when combined with the base sorbent. Inanother embodiment, the promoter is combined with an organic solventprior to reaction with the base sorbent. In another embodiment, thepromoter and optional secondary component are combined with the basesorbent substantially simultaneously. Another embodiment furthercomprises adding a mercury-stabilizing reagent selected from the groupconsisting of S, Se, H₂S, SO₂, H₂Se, SeO₂, CS₂, P₂S₅, and combinationsthereof. Another embodiment further comprises adding an optional alkalicomponent.

In an embodiment, a method is provided comprising providing a granularbase sorbent and reacting the base sorbent with a promoter selected fromthe group consisting of halogens, halides, and combinations thereof,such that the reaction product comprises a promoted sorbent effectivefor removal of mercury from a gas stream. In a further embodiment, thereaction product comprises from about 1 to about 30 grams of promoterper 100 grams of base sorbent. In another embodiment the reactionproduct has an average particle-size distribution greater than theaverage size of entrained ash particles in the gas stream from whichmercury is to be removed, such that the reaction product can besubstantially removed from the entrained ash particles by physicalmeans. In another embodiment, the reaction product has a mass meanparticle diameter greater than about 40 micrometers.

In another embodiment, the promoter is selected from the groupconsisting of molecular halogens, hydrohalides, Group V halides, GroupVI halides, and combinations thereof. In another embodiment, thepromoter is in the gas phase when contacting the base sorbent (carbon,non-carbon, or their combination). In another embodiment, the promoteris in an organic solvent when contacting the base sorbent (carbon,non-carbon, or their combination).

In another embodiment, the promoter is selected from the groupconsisting of Br₂, a Group V bromide, a Group VI bromide, andcombinations thereof.

In another embodiment, the method further comprises reacting thegranular non-carbon with an optional secondary component comprising ahalogen or a hydrohalide such that the reactivity and mercury capacityof the promoted sorbent are enhanced. In another embodiment, thepromoter and optional secondary component are contacted simultaneouslywith the non-carbon base sorbent. In another embodiment, the methodfurther comprises adding a mercury-stabilizing reagent selected from thegroup consisting of S, Se, H₂S, SO₂, H₂Se, SeO₂, CS₂, P₂S₅, andcombinations thereof. In an embodiment, a method is provided for controlof mercury in a flue gas with substantially lower sorbent requirements.Through enhanced sorbent reactivity, mercury removal per gram of sorbentis increased, thereby decreasing the capital and operating costs bydecreasing sorbent requirements.

In an embodiment, the promoted sorbent is introduced by direct injectioninto the flue gas stream. In another embodiment, the base sorbent ispromoted within the flue gas stream

In an embodiment, a method is provided for reducing mercury in flue gascomprising providing a base sorbent, either by injection or in situcreation, into a mercury-containing flue gas stream, collecting greaterthan 70 wt % of the mercury in the flue gas on the promoted sorbent toproduce a cleaned flue gas, and substantially recovering the promotedsorbent from the cleaned flue gas. In embodiments where less than 70 wt% mercury removal is desired, the required removal is attained usingless base sorbent as would be required with standard base sorbent. In afurther embodiment, the method further comprises monitoring the mercurycontent of the clean flue gas, regenerating the recovered promotedsorbent, and using the monitored mercury content of the cleaned flue gasto control the rate of base sorbent and promoter. In another embodimentthe injected promoted sorbent is prepared in-flight by reacting a basesorbent (carbon, non-carbon, or their combination) and a promoter withina pneumatic transport line from which the reaction product is injectedto the mercury-containing flue gas stream.

In another embodiment, the promoter is selected from the groupconsisting of molecular halogens, halides, and combinations thereof. Inanother embodiment, the promoter is reacted in the gas phase or as avapor. In another embodiment, the promoter is added at from about 1 toabout 30 grams per 100 grams of the base sorbent (carbon, non-carbon, ortheir combination).

In another embodiment, the injected promoted sorbent is preparedin-flight by reacting a base sorbent (carbon, non-carbon, or theircombination), a promoter, and an optional secondary component to enhancethe reactivity and capacity of the promoted sorbent within a pneumatictransport line from which the reaction product is injected to themercury-containing flue gas stream.

In another embodiment, the optional secondary component is selected fromthe group consisting of iodine, hydrohalides, Group V halides, Group VIhalides, and combinations thereof. In another embodiment, the optionalsecondary component is added at from about 1 to about 15 wt % of thepromoter content. In another embodiment, the method further comprisesadding to the promoted sorbent a mercury-stabilizing reagent selectedfrom the group consisting of S, Se, H₂S, SO₂, H₂Se, SeO₂, CS₂, P₂S₅, andcombinations thereof.

In an embodiment, the method further comprises coinjecting an optionalalkaline material, including without limitation alkaline andalkaline-earth components, to improve the efficiency of mercury captureby capturing oxidized mercury and/or capturing gaseous components thatmight otherwise reduce promoted sorbent capacity. In another embodiment,the optional alkaline material may preferably comprise calcium oxide,sodium carbonate, and the like, as are known in the art.

In another embodiment, the method further comprises using the monitoredmercury content of the cleaned flue gas to control the composition ofthe promoted sorbent. In another embodiment, the promoted sorbent isprepared in-flight by reacting a base sorbent (carbon, non-carbon, ortheir combination) and a promoter within the flue gas stream or in atransport line from which the reaction product is injected to themercury-containing flue gas stream, wherein the promoter is selectedfrom the group consisting of molecular halogens, halides, andcombinations thereof, wherein the promoter is reacted in the gas phaseor as a vapor, wherein the promoter is added at from about 1 to about 30grams per 100 grams of the base sorbent (carbon, non-carbon, or theircombination), wherein the rate at which the promoter is added and therate of promoted sorbent injection are determined by a digital computerbased, at least in part, on the monitored mercury content of the cleanedflue gas.

In an embodiment, a method for reducing the mercury content of a mercuryand ash-containing gas stream is provided wherein particulate carbonand/or non-carbon promoted sorbent with a mass mean size greater than 40μm is injected into the gas stream, mercury is removed from the gas bythe promoted sorbent particles, the promoted sorbent particles areseparated from the ash particles on the basis of size, and the promotedsorbent particles are reinjected to the gas stream. In anotherembodiment, the mercury-containing promoted sorbent particles areregenerated to remove some or substantially all of the mercury. Inanother embodiment, an alkaline component is coinjected into the gasstream. In another embodiment, the promoted sorbent may further comprisea promoter. The promoter may preferably comprise a halide, a halogen, orboth.

In an embodiment, a method for reducing mercury in a mercury-containinggas to a desired level is disclosed. The method comprises reacting acarbon base sorbent with at least one promoter selected from the groupconsisting of molecular halogens, halides, and combinations thereof toproduce a promoted carbon sorbent; allowing said promoted carbon sorbentto interact with a mercury-containing gas to capture mercury in themercury-containing gas on the promoted sorbent to produce a cleaned gas;and monitoring the mercury content of the cleaned gas. In someembodiments, the carbon base sorbent and the promoter are introducedinto the mercury-containing gas at the same location or at separatelocations. In some embodiments, the carbon base sorbent or promoter orcombination thereof is introduced into the mercury-containing gas at oneor more locations. In an embodiment, the rate at which the carbon basesorbent is introduced or the rate at which the promoter is introduced orcombination thereof is adjusted according to the monitored mercurycontent of the cleaned gas so that the mercury content of the cleanedgas is maintained at substantially the desired level with minimaloperating cost.

In a further embodiment, the method comprises reacting a non-carbon basesorbent with at least one promoter selected from the group consisting ofmolecular halogens, halides, and combinations thereof to produce apromoted non-carbon sorbent; allowing said promoted non-carbon sorbentto interact with a mercury-containing gas to capture mercury in themercury-containing gas on the promoted sorbent to produce a cleaned gas;and monitoring the mercury content of the cleaned gas. In someembodiments, the non-carbon base sorbent and the promoter are introducedinto the mercury-containing gas at the same location or at separatelocations. In some embodiments, the non-carbon base sorbent or promoteror combination thereof is introduced into the mercury-containing gas atone or more locations. In an embodiment, the rate at which thenon-carbon base sorbent is introduced or the rate at which the promoteris introduced or combination thereof is adjusted according to themonitored mercury content of the cleaned gas so that the mercury contentof the cleaned gas is maintained at substantially the desired level withminimal operating cost.

In a further embodiment, a method for reducing mercury in amercury-containing gas to a desired level is presented. The methodcomprises reacting a base sorbent with at least one promoter selectedfrom the group consisting of molecular halogens, halides, andcombinations thereof to produce a promoted sorbent, wherein said basesorbent is selected from the group consisting of a non-carbon material,a carbon material, and combination thereof; allowing said promotedsorbent to interact with a mercury-containing gas to capture mercury inthe mercury-containing gas on the promoted sorbent to produce a cleanedgas; and monitoring the mercury content of the cleaned gas.

In some embodiments, the base sorbent and the promoter are introducedinto the mercury-containing gas at the same location or at separatelocations. In some embodiments, the base sorbent or promoter orcombination thereof is introduced into the mercury-containing gas at oneor more locations. In some embodiments, introducing the base sorbent andthe promoter comprises injecting the base sorbent and the promoter intothe mercury-containing gas. In some embodiments, the promoter isintroduced into the mercury-containing gas upstream of the introductionof the base sorbent. In some embodiments, the promoter is introducedupstream of a boiler or a combustion chamber. In an embodiment, the rateat which the base sorbent is introduced or the rate at which thepromoter is introduced or combination thereof is adjusted according tothe monitored mercury content of the cleaned gas so that the mercurycontent of the cleaned gas is maintained at substantially the desiredlevel with minimal operating cost.

In an embodiment, the base sorbent for the promoted sorbent is selectedfrom the group consisting of carbon, activated carbon, porous felsicmaterials, vesicular felsic materials, porous basaltic materials,vesicular basaltic materials, clay-based compounds, alkaline compounds,calcium hydroxide compounds, sodium acetate compounds, bicarbonatecompounds, and combinations thereof. In embodiments, the non-carbonmaterial comprises Lewis basic groups and the carbon material comprisesLewis acid groups. In some cases, the non-carbon material comprisesamorphous forms of tectosilicates comprising nanoscale cavities linedwith Lewis basic oxygen associated with alkaline-earth metals. Thealkaline-earth metals comprise Group I and Group II alkaline-earthmetals. In some other cases, the non-carbon material comprises amorphousforms of phyllosilicates comprising nanoscale cavities lined with Lewisbasic oxygen.

In an embodiment, the promoted sorbent comprises metastable complexesformed between the promoter of this disclosure and inorganic species onthe non-carbon base sorbent. In some embodiments, the inorganic speciesis selected from the group consisting of sodium compounds, calciumcompounds, magnesium compounds, aluminum compounds, iron compounds, andcombinations thereof. In an embodiment, the promoted sorbent comprisesmetastable complexes formed between the promoter of this disclosure andmetal-oxygen-metal structures on the non-carbon base sorbent. In someembodiments, the promoter after being complexed with themetal-oxygen-metal structures is in the form selected from the groupconsisting of a dihalogen group, a halogen atom, a hydrohalogen group, aGroup V halide, a Group VI halide, and combinations thereof. In anembodiment, the promoted sorbent comprises activated Lewis basic groupsor activated Lewis acid groups or combination thereof. In someembodiments, the interaction between promoted sorbent and saidmercury-containing gas stream comprises mercury diffusing from the gasphase onto said promoted sorbent surface; and reacting with theactivated Lewis basic groups or activated Lewis acid groups orcombination thereof to cause chemisorption on the promoted sorbentsurface.

In an embodiment, the method for reducing mercury in amercury-containing gas to a desired level further comprises pretreatingthe base sorbent to increase the number of Lewis basic groups or Lewisacid groups or combination thereof. In some cases, the pretreatingmethods comprise chemical treatment, thermal treatment, vacuumtreatment, and combinations thereof. In some embodiments, chemicaltreatment comprises acid treatment and alkaline treatment. In anembodiment, the method for reducing mercury in a mercury-containing gasto a desired level further comprises introducing an alkali componentinto the mercury-containing gas.

As will be described in more detail below, the present invention thusprovides several advantages over previously known techniques, includingsignificantly more effective and economical mercury sorbents foreffluent gases, advantageously applicable to treating gas streams fromcombustion and gasification systems.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the preferred embodiments of thepresent invention, reference will now be made to the accompanyingdrawings.

FIG. 1 schematically illustrates methods for preparation of promotedcarbon and/or non-carbon sorbents in accordance with the presentinvention.

FIG. 2 illustrates a proposed mechanistic model of the chemicalreactions resulting in the oxidation and capture of mercury.

FIG. 3 schematically illustrates preparation of promoted carbon and/ornon-carbon sorbents and processes for flue gas mercury reduction in fluegases and/or product gases from a gasification system in accordance withthe present invention, including in-flight preparation of promotedcarbon and/or non-carbon sorbent.

FIG. 4 illustrates a mechanism for promotion of metal oxide base sorbentvia formation of a reactive halogen complex sorbent and subsequentcapture of elemental mercury on the promoted sorbent.

FIG. 5A schematically illustrates an exemplary process flow diagram forin-flight preparation of a promoted carbon and/or non-carbon sorbent.

FIG. 5B schematically illustrates an exemplary process flow diagram forin-flight preparation of a promoted carbon and/or non-carbon sorbent.

DETAILED DESCRIPTION

Herein will be described in detail specific preferred embodiments of thepresent invention, with the understanding that the present disclosure isto be considered an exemplification of the principles of the inventionand is not intended to limit the invention to that illustrated anddescribed herein. The present invention is susceptible to preferredembodiments of different forms or order and should not be interpreted tobe limited to the specifically expressed methods or compositionscontained herein. In particular, various preferred embodiments of thepresent invention provide a number of different configurations andapplications of the inventive method, compositions, and their uses.

The present invention provides a cost-effective way to capturepollutants by utilizing exceptionally reactive halogen/halide-promotedsorbents using a bromide (or other halogen/halide) treatment of thepromoted sorbent, that capture mercury via mercury-sorbent surfacereactions, at very short contact times of seconds or less. Thereactivity of the promoted sorbent toward the pollutants (i.e., mercury)is greatly enhanced, and the sorption capacity can be regenerated; i.e.,the promoted sorbent may be regenerated, recycled and/or reused.

The treated base sorbents (carbon, non-carbon, or their combination),treatment techniques, and optional additives discussed herein haveapplicability to mercury control from the product or effluent gas orgases from gasification systems, syngas generators, and othermercury-containing gas streams, in addition to the flue gas fromcombustion systems. Thus it should be understood that the termscombustion system and flue gas as used throughout this description mayapply equally to gasification systems and syngas or fuel gas, as will beunderstood by those skilled in the art.

Hereinafter the disclosure may at times discuss the use of carbon basesorbents in further details; however the use of non-carbon base sorbentsand a combination of carbon and non-carbon base sorbents is alsocontemplated to at least the same degree as carbon base sorbents.

Referring now to FIG. 1, there is shown a block flow diagramillustrating some preferred embodiments of the process of the presentinvention to prepare promoted sorbents useful for mercury capture in amercury containing gas, such as a flue gas and/or product gas fromgasification system streams. In a preferred embodiment illustrated bypath 10-20, block 10 illustrates providing a base sorbent while block 20illustrates adding a halogen or halide promoter that reacts with thebase sorbent to produce a product-promoted sorbent. In embodiments wherethe halogen or halide is added, for example, as a vapor, no furthersteps may be necessary. In embodiments where the halogen or halide isadded in, for example, a solvent, it may be desirable to employ solventremoval as illustrated by block 20A to produce a product-promotedsorbent suitable for injection.

Referring still to FIG. 1, another preferred embodiment of the processof the present invention is illustrated by path 10-20-30, comprisingproviding a base sorbent as shown by block 10, adding a halogen orhalide promoter that reacts with the base sorbent, illustrated at block20, and adding a secondary component illustrated at block 30 that reactswith the result of block 20 to produce a product-promoted sorbent. Inembodiments where both the halogen or halide promoter and the secondarycomponent are added, for example, as a vapor, no further steps may benecessary. In embodiments where the halogen or halide promoter and/orsecondary component are added in, for example, a solvent, it may bedesirable to employ solvent removal as illustrated by block 30A toproduce a product-promoted sorbent suitable for injection.

Referring still to FIG. 1, another preferred embodiment of the processof the present invention is illustrated by path 10-40, comprisingproviding a base sorbent as illustrated at block 10 and adding a halogenor halide promoter and a secondary component to the base sorbenttogether, with which they react to produce a product-promoted sorbent asillustrated by block 40. As above, in embodiments where vapor additionsare made to the base sorbent, no further steps may be desired. Inembodiments where one or more components are added in solvent, a solventremoval step may be provided as illustrated by block 40A to produce aproduct-promoted sorbent suitable for injection.

Referring still to FIG. 1, another preferred embodiment of the processof the present invention is illustrated by path 10-50 in combinationwith path 20-50. In this embodiment, a base sorbent as illustrated byblock 10 is introduced to the mercury containing gas as illustrated byblock 50 while a halogen or halide promoter as illustrated by block 20is introduced to the mercury containing gas stream. Thus the basesorbent and promoter react at block 50 to produce a product-promotedsorbent. In a similar manner, a secondary component as illustrated byblock 30 may be added to the halogen or halide promoter as illustratedby block 20 and introduced into the mercury containing gas asillustrated by block 50. In embodiments where both the halogen or halidepromoter and secondary component are added, for example, by vapor, nofurther steps may be taken. In embodiments where the halogen or halideand/or secondary component are added in, for example, a solvent, it maybe desirable to employ solvent removal as illustrated by block 20Aand/or block 30A.

Referring still to FIG. 1, also illustrated are preferred embodiments inwhich, as illustrated by block 50, a mercury containing gas stream istreated with product-promoted carbon sorbent prepared as describedabove.

In some preferred embodiments, the carbon base sorbent provided maypreferably be any of several types, as understood by those skilled inthe art. For example, the carbon base sorbent may include powderedactivated carbon, granular activated carbon, carbon black, unburnedcarbon, carbon fiber, carbon honeycomb or plate structure, aerogelcarbon film, pyrolysis char, regenerated activated carbon fromproduct-promoted carbon sorbent, or other types as known in the art.

In some preferred embodiments, the carbon base sorbent provided maypreferably have a mass mean particle size greater than the fly ash in amercury containing gas, such as a flue gas stream, to be treated.

In some preferred embodiments, the carbon base sorbent provided maypreferably have a mass mean particle diameter preferably greater than 40micrometers, more preferably greater than 60 micrometers, or aparticle-size distribution greater than that of the fly ash or entrainedash in a flue gas stream, or other mercury containing gas, to betreated, such that the activated carbon and ash can be separated byphysical means.

In some preferred embodiments, the halogen or halide promoter that isadded to, and reacts with, the carbon base sorbent may preferablycomprise, by way of illustration and not limitation, a molecular halogenin vapor or gaseous form, a molecular halogen in an organic solvent, aGroup V or Group VI halide, such as PBr₃ or SCl₂, respectively, invapor, liquid, or solution form (though not in an aqueous solvent).

Embodiments are also provided in which the organic solvent maypreferably comprise a chlorinated hydrocarbon, such as dichloromethane,a hydrocarbon solvent, including for example, petroleum ether, ligroin,pentane, hexane, toluene, and benzene, carbon disulfide, a wastesolvent, an ether, a recycled solvent, a supercritical solvent, such assupercritical CO₂, water (though not in the case of a Group V or GroupVI halide), and others as will be apparent to those of skill in the art.

Referring now to FIG. 4, a theory is illustrated developed fromscientific evidence to explain the nature of the promoting compounds.For example, as illustrated in FIG. 4, a molecular bromine moleculeforms a complex with the surface of the base sorbent comprising a highsurface area form of a metal oxide. Complexing with bromine can occur atsurface and defect sites on the surface of the glassy amorphoustectosilicates by association with the Lewis basic oxygen lining theinterstitial cavities and the alkali actions on the surface. Molecularhydrogen bromide or an electrophilic or Lewis acid bromine compoundreact to form a similar structure. The precedence for this promotion ofa metal oxide surface complex with an acidic species is described in apaper by Stark and Klabunde (Klabunde, K. J. Chem. Mater. 1996, 8,1913-1918) who showed addition of acids, HCl, SO₃, and NO, to a MgOsurface to form a surface complex. In U.S. Pat. No. 6,517,423 to Koperet al. the described surface complexes were active for destroyingbiological agents and toxins. As shown in FIG. 4, addition of halogensto a metal oxide surface (A) can form a complex that could be describedas a bromide-oxybromide species (B), in which electrophilic reactivityis maintained, owing to the formation of positive charges on thesurface. The electrophilic complex formed on the metal oxide basesorbent comprises an active site for oxidation of elemental mercury.Addition of mercury to the complex results in formation of a mercuryoxygen bond and, simultaneously, a mercury bromine bond as shown incomplex C in FIG. 4. Thus the final structure is a stable oxidized Hgform (D) described as a metaloxymercuric bromide.

In summary, it has now been found that the formation of the new bromidecomplex with the metal oxide surface increases the surface reactivitytoward mercury and other pollutants. Additionally, the resulting bromidecompound is uniquely suited to facilitate oxidation of the mercury. Theeffectiveness of the oxidation apparently results from the promotioneffect of the halide, exerted on the developing positive charge on themercury during the oxidation, known in the chemical art as a specificcatalytic effect. Thus as the mercury electrons are drawn toward thepositive surface oxygen, the halide anion electrons are pushing in fromthe other side, stabilizing the positive charge developing on themercury and lowering the energy requirement for the oxidation process.Bromide is especially reactive, owing to the highly polarizableelectrons in the outer 4p orbitals of the ion. Thus adding HBr or Br₂ tothe appropriate metal oxide forms a similar surface bromide complex, inwhich the positive oxygen oxidizes the mercury with the assistance ofthe bromide ion.

In embodiments, a non-carbon base sorbent with Lewis basic sites/groupscomprising metal-oxygen-metal structures is activated by a promoter ofthis disclosure, forming a promoted sorbent. The promoter and themetal-oxygen-metal structures of the non-carbon base sorbent formmetastable complexes, which are responsible for mercury capture viachemisorption. For example, metastable complexes may form between ahalogen promoter and inorganic species on a non-carbon base sorbent,wherein inorganic species include sodium (Na), calcium (Ca), magnesium(Mg), aluminum (Al), iron (Fe) compounds, and combinations thereof. Insome embodiments, the metastable complexes comprise a dihalogen group,such as Br—Cl, Br—Br, complexed with metal-oxygen-metal structures ofthe base sorbent. In some embodiments, the metastable complexes comprisea halogen atom, complexed with metal-oxygen-metal structures of the basesorbent. In some embodiments, the metastable complexes comprise ahydrohalogen group, complexed with metal-oxygen-metal structures of thebase sorbent. In some embodiments, the metastable complexes comprise aGroup V or Group VI halide, complexed with metal-oxygen-metal structuresof the base sorbent. In some embodiments, the metal-oxygen-metalstructures of the base sorbent are complexed with combinations of thefunctional groups disclosed herein. Without wishing to be limited by atheory, it is believed that mercury capture via chemisorption takesplace through the action of mercury oxidation provided by thesemetastable complexes.

In embodiments, a carbon base sorbent with Lewis acid sites/groupscomprising graphene sheets is activated by a promoter of thisdisclosure, forming a promoted sorbent. The promoter and the graphemesheets of the carbon base sorbent form stable compounds, which areresponsible for mercury capture via chemisorption. Without being limitedby a theory, it is believed that mercury capture via chemisorption takesplace through the action of mercury oxidation provided by these stablecompounds. As one skilled in the art would appreciate, the versatilityof chemistry associated with the base sorbent (carbon, non-carbon, orcombination thereof) and the promoter of this disclosure enablesversatile applications of the promoted sorbent system for mercurycapture. This is especially advantageous because mercury content in fluegases varies from facility to facility, from operation to operation, andfrom day to day.

Examples of non-carbon base sorbents are amorphous forms oftectosilicates that comprise nanoscale cavities lined with Lewis basicoxygen associated with Group I alkali metals and Group II alkaline-earthmetals. Such tectosilicates can be found in naturally occurringminerals, including, but not limited to, perlite and pumacite. Examplesof non-carbon base sorbents also include amorphous forms ofphyllosilicates. It is appreciated that other minerals may be used andalso treated chemically and thermally to increase the activity of thebase sorbent materials, such as phyllosilicates in the amorphous forms.

In some embodiments, bentonites are used as non-carbon base sorbents,including sodium bentonite and calcium bentonite. The use of other typesof bentonites is contemplated as is known to one skilled in the art. Theapplication of bentonite base sorbents is by introducing them into theflue gas at a location of the mercury capture system wherein thetemperature of that location is below 800° C.

In some embodiments, the base sorbents are treated chemically and/orthermally to increase their activity. For example, perlite as a basesorbent may go through vacuum treatment and then thermal treatment so asto reduce the moisture contained therein, increase its activity, andpotentially alter its morphology. Other treatment processes includechemical treatment, such acid treatment and alkaline treatment. Thesetreatment methods may be combined to achieve desired effects as known toone skilled in the art. One of the desired effects is to increase thenumber of available Lewis acid sites/groups in the carbon base sorbentand/or the number of available Lewis basic sites/groups in thenon-carbon base sorbent for subsequent activation via reaction with apromoter disclosed herein.

Referring now to FIG. 3, a schematic flow diagram is provided of mercurycontrol system 100 comprising preparation of promoted base sorbents andflue gas mercury reduction in accordance with preferred embodiments ofthe present invention. In the exemplary embodiment shown, there isprovided a base sorbent reservoir 110, a halogen/halide promoterreservoir 120, a secondary component reservoir 130, and an alkalicomponent reservoir 180, each of which with corresponding flow controldevice(s) 201, 202, 203, and 208/209, respectively. In conjunction withthe alkali component reservoir 180, flow control devices 208 and 209 canbe used independently, together, or not at all. Further, reservoirs 110,120, 130 and 180 are optional and can be used in any combination, or notat all, whereby the otherwise stored components can be introduced intothe system by other means either together or independently. Further, thealkali and secondary components may not be used at all within thesystem, if so desired.

As shown, reservoirs 110, 120, 130, and 180 connect through theirrespective flow control devices and via associated piping, to transportline 115. Alkali component reservoir 180 may also connect, throughrespective flow control devices and via associated piping, to transportline 118. A source of air, nitrogen, or other transport gas(es) isprovided by gas source 170 to transport line 115 for the purpose ofentraining materials discharged from reservoirs 110, 120, 130, and 180and injecting such materials, via injection point 116, into contaminatedflue gas stream 15. A source of air, nitrogen, or other transportgas(es) may be provided by gas source 171 to transport line 118 for thepurpose of entraining materials discharged from reservoirs 180 andinjecting such materials, via injection point 119, into flue gas stream15. Gas sources 170 and 171 may be the same or different, as desired.Alternatively, transport gas(es) may be provided to both transport lines115 and 118 by gas source 170 (connection from source 170 to line 118not shown). Although gas sources 170 and 171 are shown in FIG. 3 ascompressors or blowers, any source of transport energy known in the artmay be acceptable, as will be appreciated by those of skill in the art.

For clarity, single injection points 116 or 119 are shown in FIG. 3,although one skilled in the art will understand that multiple injectionpoints and/or locations are within the scope of the present invention.In the embodiment shown, transport line 15 comprises multiple linesallowing for multiple injections and separate and/or combined injectionsof base sorbent 110 and promoter 120 and/or 130. One mode of operation,by example, comprises providing base sorbent 110 in a common line whichis promoted inline “in-flight” using promoter 120 and/or 130 andinjected at point 116. Another mode of operation, by example, comprisestransport and injection of a base sorbent 110 in a separate line to apoint downstream of the injection of promoter 120 and/or 130 in a lineat point 116 which is upstream of injection of base sorbent 110,resulting in in-flight preparation at a promoted sorbent within stream15.

In the exemplary embodiment shown, an optional optical density measuringdevice(s) 204 is connected to transport line 115 and/or 118 to providesignals representative of the optical density inside transport line 115and/or 118 as a function of time.

Downstream from injection point 116 and 119 is provided particulateseparator 140. By way of illustration and not limitation, particulateseparator 140 may comprise one or more fabric filters, one or more ESPs,or other particulate removal devices as are known in the art. It shouldbe further noted that more than one particulate separator 140 may exist,sequentially or in parallel, and that injection point 116 and 119 may beat multiple locations upstream and/or downstream of 140 when parallel,sequential, or combinations thereof exist. Particulate separator 140produces at least a predominantly gaseous (“clean”) stream 142 and astream 141 comprising separated solid materials. A sorbent/ash separator150 separates stream 141 into a largely ash stream 152 and a largelysorbent stream 151. Stream 151 may then preferably be passed to anoptional sorbent regenerator 160, which yields a regenerated sorbentstream 161 and a waste stream 162.

An optional continuous emission monitor (hereinafter “CEM”) 205 formercury is provided in exhaust gas stream 35 to provide electricalsignals representative of the mercury concentration in exhaust stream 35as a function of time. The optional mercury CEM 205 and flow controllers201, 202, 203, 208, and 209 are electrically connected via optionallines 207 (or wirelessly) to an optional digital computer (orcontroller) 206, which receives and processes signals and preferablycontrols the preparation and injection of promoted carbon sorbent intocontaminated flue gas stream 15.

In operation, as example, promoted sorbent and/or an optional alkalicomponent is injected into contaminated flue gas stream 15. Aftercontacting the injected material with the contaminated flue gas stream15, the injected material reduces the mercury concentration,transforming contaminated flue gas into reduced mercury flue gas, 25.The injected material is removed from the flue gas 25 by separator 140,disposed of or further separated by optional separator 150, and disposedof or regenerated by an optional regenerator 160, respectively. Thereduced mercury clean flue gas stream 142 is then monitored for mercurycontent by an optional CEM 205, which provides corresponding signals toan optional computer/controller 206. Logic and optimization signals from206 then adjust flow controllers 201, 202, 203, 208, and 209 to maintainthe mercury concentration in exhaust stream 35 within desired limits,according to control algorithms well known in the art. Flow controllers201, 202, 203, 208, and 209 can also be adjusted manually or by someother automated means to maintain the mercury concentration in exhauststream 35 within desired limits, according to control algorithms wellknown in the art.

Referring still to FIG. 3, several embodiments are illustrated forpreparation and injection of promoted sorbents and/or alkali componentsin accordance with the present invention. Stream 111 provides forintroduction of base sorbent from reservoir 110, as metered by flowcontroller 201 manually or under the direction of computer 206. Thehalogen/halide may be combined and react with the base sorbent accordingto any of several provided methods. The halogen/halide may preferably becombined via line 121 directly into transport line 115, within which itcontacts and reacts with the base sorbent prior to injection point 116or downstream at point 116. This results in in-flight preparation of apromoted sorbent in accordance with the invention. Further, thehalogen/halide may be combined via line 121 b with the base sorbentprior to entering transport line 115. Still further, the halogen/halidemay be contacted and react with the base sorbent by introduction vialine 121 c into reservoir 110. This option is preferably employed when,for example, reservoir 110 comprises an ebulliated or fluidized bed ofbase sorbent, through which halogen/halide flows in gaseous form or as avapor. Of course, the halogen/halide may also preferably be contactedwith the base sorbent in liquid form or in a solvent, as discussedpreviously, and solvent removal (not shown in FIG. 3) may then beprovided if necessary as mentioned with respect to embodiments discussedwith reference to FIG. 1.

Similarly, the optional secondary component may be contacted and reactdirectly in transport line 115 via line 131, or optionally as describedabove with respect to the halogen/halide, via lines 131 b and 131 c.

Similarly, the optional alkali component from 180 may either be added intransport line 115 directly, or may be injected separately by transportline 118, combining downstream in flue gas 15 for synergistic effectswith the base sorbent, promoted sorbent, or optional secondarycomponents. Being able to vary on-site the amount of the optional alkalicomponent relative to base sorbent, promoted sorbent, or optionalsecondary components is a key feature to overcome and optimize forsite-specific operating and flue gas conditions.

In some preferred embodiments wherein contacting between components andreaction is performed in a liquid or solvent phase, stirring of suchliquid and/or slurry mixtures may be provided. In other embodiments, thehalogen/halide promoter and optional secondary component(s) maypreferably be sprayed in solution form into or on the base sorbent. Insome such embodiments, drying, filtering, centrifugation, settling,decantation, or other solvent removal methods as are known in the artmay then be provided.

In embodiments wherein the halogen/halide promoter is in gaseous orvapor form, it may be diluted in air, nitrogen, or other gas asappropriate. The halide/halogen gas, for example, gaseous HBr or Br₂,may be passed through an ebulliated or fluidized bed of granular orfibrous base sorbent, with the promoted sorbent so produced removed fromthe top of the bed via gas entrainment for injection.

In some embodiments, the secondary component(s) may preferably compriseiodine or other halogens, hydrohalides, including without limitation HI,HBr, HCl, a Group V or Group VI element with a molecular halogen, suchas SCl₂ and others. In some preferred embodiments, the promoted sorbentmay comprise from about 1 to about 30 g of halogen/halide per 100 g ofbase sorbent. In some preferred embodiments, the promoted sorbent maycomprise a secondary component in concentration of from about 1 to about15 wt % of the concentration of the halogen/halide component.

In still other embodiments, the product-promoted sorbent may be appliedto a substrate. In other embodiments, such prepared substrate(s) may becaused to contact a contaminated flue gas or gasification system productgas stream for mercury reduction purposes. Such substrates may bemonolithic, rotating, or exposed to the gas stream in any number of waysknown to those skilled in the art.

In some embodiments, a method is provided whereby a mercury stabilizingreagent is added to a promoted sorbent to produce a bifunctionalsorbent. Such stabilizing reagent(s) may be sequentially added, eitherbefore or after the addition and reaction of the halogen/halide. In somepreferred embodiments, the halogen/halide preferably comprises Br orHBr, and the mercury-stabilizing reagent may comprise S, Se, H₂S, SO₂,H₂Se, SeO₂, CS₂, P₂S₅, and combinations thereof.

Halogens in Mercury Capture

Methodologies for using halogens for the treatment of flue gas have beenproblematic, owing to their reactivity with other gases and metals,resulting in corrosion and health issues. A “halogen” is defined as amember of the very active elements comprising Group VIIA (CASnomenclature is used throughout; Group VIIA (CAS) corresponds to GroupVIIB (IUPAC)) of the periodic table. In the molecular elemental form ofthe halogens, including F₂, Cl₂, Br₂, and I₂, the reaction with hot fluegas components leaves little to react with elemental mercury. The atomicelemental halogen form, which includes the fluorine, chlorine, bromine,and iodine atoms, is about a million times more reactive to mercury, butthe concentration of the atomic forms is typically extremely low. In alarge portion of electric utility coal combustion facilities, theconcentrations are generally not sufficient to oxidize a significantamount of mercury.

The term “halide” as used herein is defined as a compound formed fromthe reaction of a halogen with another element or radical. In general,halide compounds are much less reactive than the molecular halogens,having a low chemical potential. Halides are considered reduced formsthat do not, alone, oxidize other compounds. In the conventional view,therefore, a halide salt-treated sorbent will not effectively oxidizeelemental mercury and capture elemental mercury.

Halogen-Promoted Sorbent Characteristics

The promoted sorbent described here has a very high initial reactivityfor oxidizing mercury and therefore, can be used in very small amountsto achieve very high capture efficiencies, thus lowering operation costsand lessening waste disposal problems. In addition, further disposalreductions are obtainable by regenerating and reusing the promotedsorbents produced using the inventive technology. The time intervalrequired for the mercury and the promoted sorbents of the presentinvention to successfully interact in a flue gas duct, with subsequentcollection of the mercury on the promoted sorbent and ash, is veryshort—less than seconds. Clearly, such collection times require thepromoted sorbent to have both high capacity and high reactivity towardmercury. The promoted sorbent can be utilized in a very finely powderedform to minimize mass-transfer limitations. However, again, thereactivity should be very high to capture all of the mercury encounteredby the fine particles. Additionally, use of these enhancementtechnologies allows capture to be effective for larger sorbent particleswhich also allows separation of the promoted sorbent from the ash toenable subsequent regeneration as well as ash utilization. One featureof this invention is the process to prepare a promoted sorbentcontaining a halide component formed on and/or within the base sorbentstructure that provides a sorbent that is highly active on initialcontact with the mercury-contaminated gas stream, which allows for veryeffective capture of the mercury.

The inventive sorbents chemically combine bromine species with Lewisacid/basic sites on the base sorbent. For example, x-ray photoelectronspectroscopy has established that the addition of bromine, chlorine,HBr, or HCl formed a chemical compound in the carbon structure. Thus thepromoted sorbent produced from halogen and base sorbent does notrepresent a molecular halogen form, but rather a new chemically modifiedstructure. This phenomenon may not occur with the less reactive iodine,where an I₂ molecular complex can exist on the carbon basal plane. Inthe case of bromine, modified cationic carbon has a high chemicalpotential for oxidation of mercury. Thus an entirely new model ispresented for the reactivity of the bromine-treated carbon with mercuryshown in FIG. 2. The reactive carbon form can preferably be generated bythe addition of bromine, hydrogen bromide, or combinations of bromineand other elements, as described herein. Halogen treatment resulted inhigher-activity carbons because the halide anions (especially bromideand iodide) were effective in promoting oxidation by stabilizing thedeveloping positive charge on the mercury in the transition state foroxidation. Based on this model, several innovative, inexpensive,activity-enhancing features have been developed.

Optional Second Component

It has been demonstrated that addition of an optional second component,in addition to the bromine, results in improved reactivity and capacityfor the promoted sorbent, typically exceeding that of both the untreatedbase sorbent and the brominated carbon. The second compound compriseseither a second halogen or a compound derived from a second halogen,such as HI. Thus in addition to having a reactive carbon form present,the second component generates a Lewis base with greater ability tostabilize the developing positive charge on the mercury. Thus the secondcomponent is an element with more polarized electrons (4p and 5p).

Optional Alkali Component

It has been demonstrated that addition of an optional alkali componentwith a base or promoted activated carbon results in improved mercurycapture, typically exceeding that of both the untreated carbon and thepromoted carbon. Test data indicate that flue gas contaminants, flue gasconstituents (SO₂, NO_(x), HCl, etc), operating temperature, mercuryform, and mercury concentration may impact the effectiveness of thealkali addition. This suggests the need to be able to adjust and tailorthe alkali-to-activated-carbon ratio on-site in order to overcome andoptimize for a given set of site conditions.

The synergy that can be gained when coinjecting the two materials can beexplained as follows. First, testing shows that binding sites onactivated carbon (hereinafter “AC”) can be consumed by chlorine species,sulfur species (i.e., sulfates), and other flue gas contaminants(arsenates, selenates, etc). The addition of optional alkali materialwill interact and react with these species/contaminants, thus minimizingtheir consumption of AC mercury binding sites. Second, testing alsoshows that standard AC will continue to oxidize mercury, even though thebinding sites are fully consumed. This oxidized mercury can then reactwith alkali material and subsequently be captured by particulate controldevices. Consequently, the addition of the optional alkali componentacts to protect mercury-binding sites and capture oxidized mercury,thereby resulting in improved mercury reduction at lower cost. Alkali isgenerally much lower in cost (˜ an order of magnitude less) than AC;thus more of it can be used, still resulting in overall lower costs.

In-Flight Sorbent Preparation

As stated previously, the halogen promoted sorbent can be readilyproduced in-flight. This is accomplished by, for example, contacting thevapors of any combination of halogens and, optionally, a secondcomponent, in-flight, with base sorbent particles. The particles may bedispersed in a stream of transport air (or other gas, such as the fluegas itself), which also conveys the halogen/halide-promoted sorbentparticles to the flue gas duct, or other contaminated gas stream, fromwhich mercury is to then be removed. There is no particular temperaturerequirement for this contact. This technology is obviously very simpleto implement, and results in a great cost savings to facilities usingthis technology for mercury capture.

Referring to FIGS. 5A and 5B, process flow diagrams are shown asexamples of the process for mercury removal from a mercury containinggas, such as a flue gas. In an embodiment shown at FIG. 5A, gas inletstream 501 passes through chamber 301 and enters air heater 302 asstream 502; then it exits air heater 302 as stream 503 and passesthrough particulate control device 303 and enters scrubber 304 as stream504; finally it exits scrubber 304 as stream 505. As shown, chamber 301is a boiler, however, one skilled in the art will appreciate thatchamber 301 can also be the combustion chamber of a coal fired boiler, astand alone combustion chamber or any other chamber in which mercurycontaining gas is either generated or passed through. Injection streams401, 402, 403 and 404, in addition to stream 501, are multiple locationswherein promoter, base sorbent, or a combination of promoter and basesorbent may be introduced.

FIG. 5B shows an embodiment similar to that of FIG. A, but with theposition of the scrubber and particulate control device switched.Specifically, gas inlet stream 501′ passes through chamber 301′ andenters air heater 302′ as stream 502′; then it exits air heater 302′ asstream 503′ and passes through scrubber 304′ and enters particulatecontrol device 303′ as stream 504′; finally it exits particulate controldevice 303′ as stream 505′. As shown, chamber 301 is a boiler, however,one skilled in the art will appreciate that chamber 301 can also be thecombustion chamber of a coal fired boiler, a stand alone combustionchamber or any other chamber in which mercury containing gas is eithergenerated or passed through. Injection stream locations 401′, 402′, 403′and 404′, in addition to stream 501′, represent multiple locationswherein promoter, base sorbent, or a combination of promoter and basesorbent may be introduced.

At each of the aforementioned injection stream locations 401, 401′, 402,402′, 403, 403′, 404 and 404′, 501, 501′ multiple injection points arecontemplated so that promoter and base sorbent may be injected as asingle injection stream or as separate injection streams, as furtherillustrated in the following exemplary embodiments.

In an embodiment, a promoter is introduced to chamber 301 at location401. A base sorbent (carbon, non-carbon, or their combination) isintroduced at location 402 upstream of air heater 302. In a furtherembodiment, a promoter is introduced to at location 402 upstream of airheater 302. A base sorbent (carbon, non-carbon, or their combination) isalso introduced at location 402 either as a separate stream or as amixed single stream with the promoter. In another embodiment, a promoteris introduced at locations 401 and 402. A base sorbent (carbon,non-carbon, or their combination) is introduced at location 402 eitheras a separate stream or as a mixed single stream with the promoter.

With the aid of this disclosure, one of ordinary skill in the art willbe able to configure the process in many different fashions for mercuryremoval using the promoted sorbent. All these configurations areconsidered equivalents of the disclosed process and therefore are withinthe scope of the claimed invention.

Advantages of On-Site Preparation

In-flight preparation of the halogen/halide-promoted sorbent on locationproduces certain advantages. For example, the treatment system can becombined with the base sorbent injection system at the end-use site.With this technique, the halogen/halide is introduced to the basesorbent air (or other gas, including to the flue gas or other mercurycontaining gas) mixture in a transport line (or flue gas duct part ofthe base sorbent storage and injection system). This provides thefollowing benefits over current conventional concepts for treatingsorbents off-site:

-   -   Capital equipment costs at a treatment facility are eliminated.    -   Costs to operate the treatment facility are eliminated.    -   There are no costs for transporting carbon and additive to a        treatment facility.    -   The inventive process uses existing hardware and operation        procedures.    -   The inventive technology ensures that the sorbent is always        fresh and, thus, more reactive.    -   No new handling concerns are introduced.    -   There are no costs for removing carbon from treatment system.    -   The inventive process allows rapid on-site tailoring of        additive-sorbent ratios in order to match the requirements of        flue gas changes, such as may be needed when fuels are changed        or loads are reduced, thus further optimizing the economics.    -   The inventive technology reduces the amount of spent sorbents        that are disposed.

With the foregoing and other features in view, there is provided, inaccordance with the present invention, embodiments including a processfor preparing and regenerating halogen/halide-promoted sorbents, whoseactivity for mercury capture is enhanced by the addition of halogen(e.g., bromine) to the base sorbent structure.

Sorbent Injection Location

Some of the preferred embodiments contemplate the use of ahalogen-promoted sorbent in a powdered form that has been injected intoa flue gas stream before or after ash particulates have been removed.Other embodiments of the inventive composition of the halogen-promotedsorbent comprise a powdered modified AC prepared by adding Br₂ or HBrplus a second optional component. Other embodiments allow the additionof the optional alkali component in conjunction with a base AC and/orwith the use of a halogen-based sorbent and any other combinations ofthe sorbent technologies provided in this patent. Other embodimentsallow for in-flight preparation of promoted sorbents by using andcombining the promoters and base sorbents. Alternatively, embodimentsinclude methods wherein the base sorbent is on a moving contactorconsisting of particles or fibers containing one or more of thecompositions listed above.

Sorbent Regeneration

Any of the above embodiments of the halogen/halide-promoted carbonand/or non-carbon sorbent can be easily regenerated; the poisoningcontaminants from the flue gas are preferably removed, and aninexpensive promoting agent added, to restore mercury sorption activity.This process of promoting the activity of the carbon and/or non-carbonitself contrasts with the earlier, more expensive, conventional methodsof adding a reagent (such as peroxide, gold, triiodide, etc.) to a basesorbent. The halogen/halide-promoted carbon sorbent of the presentinvention, treated with bromine and/or optional components, isnoncorrosive. Detailed examples of sorbent regeneration techniques aredescribed in copending, commonly owned PCT Patent Application No.PCT/US04/12828, titled “PROCESS FOR REGENERATING A SPENT SORBENT,” whichis hereby incorporated by reference in its entirety.

Sorbent Injection Control Schemes

Another advantage of the present invention relates to the use of afeedback system to more efficiently utilize certain aspects of theinvention. Where possible and desirable, the mercury control technologyof the present invention may preferably utilize continuous measurementof mercury emissions as feedback to assist in control of the sorbentinjection rate. Tighter control on the sorbent and optional component(s)levels can be achieved in this way, which will ensure mercury removalrequirements are met with minimal material requirements, thus minimizingthe associated costs. In an embodiment, the mercury emissions arecontinuously measured downstream of the injection location, preferablyin the exhaust gas at the stack.

EXAMPLES

To more clearly illustrate the present invention, an example ispresented below. This example is intended to be illustrative, and nolimitations to the present invention should be drawn or inferred fromthe example presented herein.

Example 1 Non-Carbon-Promoted Sorbent Tests at Full-Scale Plants

Full-scale commercial tests were conducted at several coal-burningfacilities equipped with ESPs wherein three types of pre-cursers wereinjected upstream of an ESP at 325° F. In all, more than thirty testswere performed with variations in base sorbent material, base sorbentmaterial injection rates and promoter injection rates. Promoters werehalogen-based materials. Base sorbents were clay-based materials such asperlite and pumacite, which are vesicular forms of tectosilicatescomprising silicates and aluminate tetrahedra with alkali metals in theinterstitual spaces. Each base sorbent was injected upstream from theESP.

The flue gas flow rate for the test was approximately 23 milliondsft³/hr with a corresponding mercury flow rate of 0.0097 lbs Hg/hr.Initial mercury concentration in the flue gas prior to the introductionof the base sorbent or promoted sorbent was approximately 0.000417 lbsHg/million dsft³.

As shown in Table 1, each base sorbent was injected at three differenttest rates: about 100 lb/hr (“Low”), about 150 lb/hr (“Ave.”) and about200 lb/hr (“High”). At each test rate, the mercury removal rate wasmeasured both with and without the use of a promoter to show therelative benefit of introducing the promoter into the flue gas stream.For the tests where promoter was injected into the flue gas stream,promoter was injected at about 20 lb/hr for the “Low” tests, at about 25lb/hr for the “Ave.” tests and from about 30 to about 50 lb/hr for the“High” tests. The percent mercury removal for each test was calculatedbased on measurements taken from the inlet flue gas and the outlet fluegas. The results as shown in Table 1 clearly show a significantimprovement in mercury capture when a promoter and base sorbent areinjected, as compared to the injection of a base sorbent alone.

TABLE 1 Test Data for Non Carbon-Promoted Sorbents Base Base SorbentInjection Promoter Injection Percent Removal Sorbent Rate (lb/hr) Rate(lb/hr) of Mercury (%) Material Low Ave. High Low Ave. High Low Ave.High Perlite 100 150 200 0 0 0 37 40 43 100 150 200 20 25 35 55 72 74Pumacite 100 150 200 0 0 0 25 25 25 100 150 200 20 25 50 52 60 65 Clay-100 150 200 0 0 0 50 52 54 Based Mix 100 150 200 20 25 30 58 65 74 ofSilica and Alumina

While the preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the invention. Theembodiments described herein are exemplary only and are not intended tobe limiting. Many variations and modifications of the inventiondisclosed herein are possible and are within the scope of the invention.Accordingly, the scope of protection is not limited by the descriptionset out above, but is only limited by the claims which follow, thatscope including all equivalents of the subject matter of the claims.

The examples provided in the disclosure are presented for illustrationand explanation purposes only and are not intended to limit the claimsor embodiment of this invention. While the preferred embodiments of theinvention have been shown and described, modifications thereof can bemade by one skilled in the art without departing from the spirit andteachings of the invention. Process criteria, equipment, and the likefor any given implementation of the invention will be readilyascertainable to one of skill in the art based upon the disclosureherein. The embodiments described herein are exemplary only and are notintended to be limiting. Many variations and modifications of theinvention disclosed herein are possible and are within the scope of theinvention. Use of the term “optionally” with respect to any element ofthe invention is intended to mean that the subject element is required,or alternatively, is not required. Both alternatives are intended to bewithin the scope of the invention.

The discussion of a reference in the Background is not an admission thatit is prior art to the present invention, especially any reference thatmay have a publication date after the priority date of this application.The disclosures of all patents, patent applications, and publicationscited herein are hereby incorporated herein by reference in theirentirety, to the extent that they provide exemplary, procedural, orother details supplementary to those set forth herein.

Although the invention is described herein as a promoted sorbentmaterial and associated processes for its preparation and use, it isnevertheless not intended to be limited to the details described, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

What is claimed is:
 1. A method for separating mercury from a mercury containing gas comprising: (a) providing a carbon sorbent material and a non-carbon sorbent material, wherein the carbon sorbent material comprises activated carbon; (b) providing a halogen or halide promoter; (c) promoting at least a portion of the carbon sorbent material and the non-carbon sorbent material by chemically reacting the carbon sorbent material and the non-carbon sorbent material with the halogen or halide promoter to form a promoted halogenated carbon sorbent and a promoted halogenated non-carbon sorbent; (d) chemically reacting elemental mercury in the mercury containing gas with the promoted halogenated carbon sorbent and the promoted halogenated non-carbon sorbent to form a mercury/sorbent chemical composition; and (e) separating particulates from the mercury containing gas to form a cleaned gas, the particulates including ash and the mercury/sorbent chemical composition.
 2. The method of claim 1, further comprising the step of injecting the non-carbon sorbent material and the halogen or halide promoter into the mercury containing gas.
 3. The method of claim 2, wherein said halogen or halide promoter and the non-carbon sorbent material are injected into the mercury containing gas at the same location.
 4. The method of claim 2, wherein said halogen or halide promoter and the non-carbon sorbent material are injected into the mercury containing gas at separate locations.
 5. The method of claim 4, wherein said halogen or halide promoter is injected into the mercury-containing gas upstream of the injection of said non-carbon sorbent material.
 6. The method of claim 5, wherein said halogen or halide promoter is injected into a combustion chamber that produces a mercury-containing gas, and the non-carbon sorbent material is injected downstream of the combustion chamber.
 7. The method of claim 6, wherein said halogen or halide promoter is additionally injected downstream of the combustion chamber.
 8. The method of claim 6, wherein said combustion chamber is a boiler and the mercury-containing gas is a flue gas.
 9. The method of claim 3, wherein said halogen or halide promoter and non-carbon sorbent material are injected downstream of a chamber that produces a mercury-containing gas.
 10. The method of claim 9, wherein said chamber is a boiler and the mercury-containing gas is a flue gas.
 11. The method of claim 2, wherein the rate at which said non-carbon sorbent material is injected or the rate at which said promoter is injected or combination thereof is adjusted according to a monitored mercury content in the cleaned gas so that the mercury content of the cleaned gas is maintained at substantially a desired level.
 12. The method of claim 1, wherein said non-carbon sorbent material is selected from the group consisting porous felsic materials, vesicular felsic materials, porous basaltic materials, vesicular basaltic materials, clay-based compounds, alkaline compounds, calcium hydroxide compounds, sodium acetate compounds, bicarbonate compounds, and combinations thereof.
 13. The method of claim 12, wherein said non-carbon sorbent material is a material that reacts with oxidized mercury in the mercury-containing gas to form a second mercury/sorbent chemical composition.
 14. The method of claim 1, wherein said non-carbon sorbent material comprises Lewis basic groups.
 15. The method of claim 12, wherein said non-carbon sorbent material comprises amorphous forms of tectosilicates comprising nanoscale cavities lined with Lewis basic oxygen associated with alkaline-earth metals.
 16. The method of claim 15, wherein said alkaline-earth metals comprise Group I and Group II alkaline-earth metals.
 17. The method of claim 1 wherein said non-carbon sorbent material comprises amorphous forms of phyllosilicates comprising nanoscale cavities lined with Lewis basic oxygen.
 18. The method of claim 1, wherein said promoted non-carbon sorbent comprises metastable complexes formed between said promoter and inorganic species on the non-carbon sorbent.
 19. The method of claim 18 wherein said inorganic species is selected from the group consisting of sodium compounds, calcium compounds, magnesium compounds, aluminum compounds, iron compounds, and combinations thereof.
 20. The method of claim 1, wherein said promoted non-carbon sorbent comprises metastable complexes formed between said promoter and metal-oxygen-metal structures on the non-carbon sorbent.
 21. The method of claim 20, wherein said promoter after being complexed with the metal-oxygen-metal structures is in the form selected from the group consisting of a dihalogen group, a halogen atom, a hydrohalogen group, a Group V halide, a Group VI halide, and combinations thereof.
 22. The method of claim 1, wherein said promoted non-carbon sorbent comprises activated Lewis basic groups or activated Lewis acid groups or combination thereof.
 23. The method of claim 1, wherein said interaction between promoted non-carbon sorbent and said mercury-containing gas stream comprises: mercury diffusing from the gas phase onto a surface of said non-carbon promoted sorbent; and reacting with activated Lewis basic groups or activated Lewis acid groups or combination thereof to cause chemisorption on a sorbent surface.
 24. The method of claim 1 further comprising pretreating said non-carbon sorbent to increase the number of Lewis basic groups or Lewis acid groups or combination thereof on said sorbent.
 25. The method of claim 24, wherein pretreating said non-carbon sorbent comprises chemical treatment, thermal treatment, vacuum treatment, and combinations thereof.
 26. The method of claim 25, wherein said chemical treatment comprises acid treatment and alkaline treatment.
 27. The method of claim 1 further comprising introducing an alkali component into the mercury-containing gas.
 28. The method of claim 1, wherein said carbon sorbent or said promoter or combination thereof are introduced into the mercury-containing gas at one or more locations.
 29. The method of claim 28, wherein the rate at which said carbon sorbent is introduced or the rate at which said promoter is introduced or combination thereof is adjusted according to a monitored mercury content in the cleaned gas so that the mercury content of the cleaned gas is maintained at substantially a desired level.
 30. The method of claim 1, where the promoted halogenated carbon sorbent and the promoted halogenated non-carbon sorbent comprise about 1 to about 30 grams of the halogen or halide promoter per 100 grams of the carbon sorbent and the non-carbon sorbent. 